Dynamized interspinal implant

ABSTRACT

An interspinous process having a general horseshoe shape, a cushion element within the horseshoe and a porous coating on its outer surface.

BACKGROUND OF THE INVENTION

The leading cause of lower back pain arises from rupture or degenerationof lumbar intervertebral discs. Pain in the lower extremities is causedby the compression of spinal nerve roots by a bulging disc, while lowerback pain is caused by collapse of the disc and by the adverse effectsof articulation weight through a damaged, unstable vertebral joint.

In some cases, when a patient having a collapsed disc moves in extension(e.g., leans backward), the posterior portion of the annulus fibrosis orfolding of the ligamentum flavum may further compress and extend intothe spinal canal. This condition, called “spinal stenosis”, narrows thespinal canal and causes impingement of tissue upon the spinal cord,thereby producing pain.

There have been numerous attempts to provide relief for theseafflictions by providing a spacer that inserts between adjacent spinousprocesses present in the posterior portion of the spinal column. Thisspacer essentially lifts the upper spinous process off of the lowerspinous process, thereby relieving stenosis. In general, theseinterspinous implants are adapted to allow flexion movement in thepatient, but resist or limit extension.

U.S. Pat. No. 6,068,630 (“Zuchermann”) discloses a spinal distractionimplant that alleviates pain associated with spinal stenosis byexpanding the volume in the spinal canal or neural foramen. Zuchermanndiscloses a plurality of implants having a body portion and lateralwings. The body portion is adapted to seat between the adjacent spinousprocesses, while the wings are adapted to prevent lateral movement ofthe body portion, thereby holding it in place between the adjacentspinous processes.

U.S. Pat. No. 5,645,599 (“Samani”) attempts to relieve spinal stenosisby essentially inserting a flexible horseshoe-shaped device between theadjacent spinous processes. Although the Samani device desirablyprovides a self-limiting flexibility, it nonetheless suffers from someinadequacies. For example, the Samani device does not provide fornatural physiologic rotational movement, nor for post-operativeadjustment. In addition, the Samani device discloses the insertion of abearing cushion, and the adhesive bonding of the bearing cushion to thehorseshoe element. However, it is believed that mere adhesive bonding ofthese elements would cause the cushion to be prone to migration.

SUMMARY OF THE INVENTION

The present inventors have developed a number of flexible interspinousdevices having a number of desirable features providing improvedperformance over conventional solutions.

In a first embodiment, the device has a flexible anterior wall having anarrowed portion. The narrowed portion allows the device to twist inresponse to spinal rotation, thereby more closely mimicking naturalphysiologic movement.

Therefore, in accordance with the first embodiment of the presentinvention, there is provided an interspinous implant for insertionbetween adjacent spinous processes, the implant comprising:

-   a) a flexible body comprising:    -   i) an upper posterior portion having an upper surface adapted to        bear upon an upper spinous process,    -   ii) a lower posterior portion having a lower surface adapted to        bear upon a lower spinous process, and    -   iii) an arcuate, flexible anterior wall connecting the upper and        lower portions,        wherein the anterior wall has a narrowed portion.

In a second embodiment, the device has a cushion portion interdigitatedwith each of the upper and lower bearing portions. Because the cushionportion is interdigitated with these elements, a tenacious bond isprovided and migration concerns are alleviated.

Therefore, in accordance with the second embodiment of the presentinvention, there is provided an interspinous implant for insertionbetween adjacent spinous processes, the implant comprising:

-   a) a flexible body comprising:    -   i) an upper posterior portion having an upper surface adapted to        bear upon an upper spinous process,    -   ii) a lower posterior portion having a lower surface adapted to        bear upon a lower spinous process,    -   iii) an arcuate, flexible anterior wall connecting the upper and        lower portions, and-   b) a cushion element having an upper surface and a lower surface,    wherein the lower surface of the upper portion of the flexible body    comprises a porous coating thereon, and wherein the upper surface of    the cushion element is interdigitated with the porous coating.

In a third embodiment, the device is adapted to be post-operativelyadjustable. The adjustability allows the device to respond to an alteredphysiologic state, such as an increased collapse of the disc space ordecreased patient flexibility, by adjusting the overall stiffness of theimplant.

Therefore, in accordance with the third embodiment of the presentinvention, there is provided an interspinous implant for insertionbetween adjacent spinous processes, the implant comprising:

-   -   a) a flexible body comprising:        -   i) an upper posterior portion having an upper surface            adapted to bear upon an upper spinous process,        -   ii) a lower posterior portion having a lower surface adapted            to bear upon a lower spinous process,        -   iii) an arcuate, flexible anterior wall connecting the upper            and lower posterior portions, and        -   iv) means for adjusting the stiffness of the implant.

DESCRIPTION OF THE FIGURES

FIG. 1 a is a posterior view of the first embodiment of the interspinousimplant in the coronal plane

FIG. 1 b is a side view of the first embodiment of the interspinousimplant in the saggital plane.

FIG. 1 c is a posterior view of the first embodiment of the interspinousimplant implanted between adjacent vertebrae.

FIG. 2 a is a posterior view of the second embodiment of theinterspinous implant.

FIG. 2 b is a side view of the second embodiment of the interspinousimplant in the saggital plane.

FIG. 2 c is a side view of an embodiment of the interspinous implantimplanted between adjacent vertebrae.

FIG. 3 a is a posterior view of the third embodiment of the interspinousimplant.

FIG. 3 b is a side view of the third embodiment of the interspinousimplant in the saggital plane.

FIGS. 4 a-4 c are perspective, longitudinal and frontal cross-sectionalviews of a fourth embodiment of the present invention.

FIG. 4 d is a side view of the fourth embodiment of the interspinousimplant implanted between adjacent vertebrae.

FIGS. 5 a-b are side views of a fifth embodiment of the interspinousimplant having outer and inner flexible shells.

FIG. 6 is a side view of a functional spinal unit of the human anatomy.

DETAILED DESCRIPTION OF THE FIGURES

For the purposes of the present invention, the term “interspinous”refers to the volume located between two adjacent spinous processes ofadjacent vertebrae. The terms “anterior” and “posterior” are used asthey are normally used in spinal anatomy. Accordingly, the “anterior”portion of the interspinous device is that portion rests relativelyclose to the spinal cord, while the “posterior” portion of theinterspinous device is that portion rests relatively close to the skinon the patient's back. Now referring to FIG. 6, there is provided ananatomic “functional spinal unit” or FSU comprising an upper vertebraeVu having an upper vertebral body VB_(U) and an upper spinous processSPu, a lower vertebra having a lower vertebral body VB_(L) having alower spinous process SP_(L). The vertebral bodies lies in the anteriorA portion of the FSU, while the spinous processes lie in the posteriorportion P of the FSU. Disposed between the vertebral bodies is a discspace DISC. Disposed between the spinous process is an “interspinousregion”. Disposed between the spinous process and the vertebral body ofeach vertebra is a lamina L. The supraspinous ligament SSL liesposterior to the spinous processes. The Posterior longitudinal ligamentPLL lies posterior to the vertebral bodies.

Now referring to FIGS. 1 a and 1 b, there is provided an interspinousimplant 1 for insertion between adjacent spinous processes, the implantcomprising:

-   -   a) a flexible body 11:        -   i) an upper posterior portion 21 having an upper surface 23            adapted to bear upon an upper spinous process and a width            W_(UP),        -   ii) a lower posterior portion 31 having a lower surface 33            adapted to bear upon a lower spinous process and a width            WL_(P), and        -   iii) a flexible arcuate anterior wall 4 i connecting the            upper and lower portions and having a narrowed portion 43            defining a minimum width W_(DW),            wherein the minimum width of the anterior wall is less than            the width W_(UP) of the upper portion.

Now referring to FIG. 1 c, in use, the implant of FIGS. 1 a and 1 b isinserted into the interspinous region of an functional spinal unit(FSU), that is, between the adjacent spinous processes. The U-shapedbody has a stiffness and geometry adapted to provide the desired spacingbetween the upper and lower process. In addition, in preferredembodiments, the U-shaped body is adapted to be somewhat flexible, sothat it somewhat restricts the extent of extension motion of the FSU.

In preferred embodiments, the flexible body is U-shaped. In otherembodiments, the flexible body has a posterior wall (preferably,arcuate) that flexibly connects the posterior portions of the upper andlower bearing surfaces of the flexible body to provide an overallsubstantially oval shape.

Preferably, the flexible body has a configuration and is made of amaterial that provides a first stiffness that limits the range of motionof the FSU. In some embodiments, the flexible body stiffness provides atleast 50% of the overall initial stiffness of the implant, preferably atleast 75%, more preferably at least 90%.

Preferably, the flexible body is adapted to provide a stiffness ofbetween 50 N/mm and 1000 N/mm, more preferably between 100 N/mm and 500N/mm. When the flexible body stiffness is in this range, it maintainsthe flexion/extension ROM of a normal lumbar FSU to less than 20degrees, with less than 13 degrees of motion in flexion and less than 7degrees of motion in extension. Preferably, the typical displacement ofthe posterior ends of the device under physiologic loading in thesaggital plane is in the range of 1-6 mm.

The flexible can be made of a suitable biocompatible material typicallyused in structural spinal applications, including metals, plastics andceramics. In some embodiments, the flexible body is made of a materialselected from the group consisting of titanium alloy (including memorymetals and superelastic alloys), stainless steel, and chrome cobalt.Preferably, the flexible body is provided in a sterile form.

Now referring to FIG. 1, in some embodiments, the flexible body has aheight H of between 10 mm and 20 mm; a thickness T of between 1 mm and 2mm; a length L of between 20 mm and 30 mm, and a width W of between 3and 20 mm, preferably between 5 mm and 10 mm. Thus, in some embodiments,the height of the flexible body is as least as great as its width. Inthese embodiments, the implant can be easily inserted between typicaladjacent spinous processes.

In some embodiments, the flexible body has a longitudinal cross sectionhaving a horseshoe shape. In others, the longitudinal cross-sectiondescribes a circle. In others, it is a pill shape. In others, it issubstantially oval. In some embodiments, the upper and lower posteriorportions are substantially non-parallel.

In some embodiments, as shown in FIG. 1 b, the upper and lower posteriorportions of the flexible body each have a longitudinal recess 25defining a bearing surface 23, 33 and opposing recess walls 27. Therecess shape is adapted to receive projecting portions of the opposedspinous processes, thereby securing the U-shaped shell between thespinous processes. In some embodiments, the recess walls have teeth 28extending inwardly therefrom that provide a more grip upon the spinousprocesses. In some embodiments, at least the bearing surfaces of therecess have teeth 415 (as shown in FIG. 4 c) extending outwardlytherefrom that provide a more grip upon the spinous processes.

In some embodiments, the recess 25 defines an upper pair of extensions45 extending from the bearing surface 33 and collectively defining abracket. Each extension may comprise a transverse throughhole (notshown) adapted for fixing the implant to the adjacent spinous processes.

In some embodiments, each extension comprises a transverse throughholeadapted for fixing the implant to the adjacent spinous processes. Insome embodiments, the implant further comprises a fastening elementhaving a first end extending through the first transverse throughole anda second end extending through the second transverse through-hole.

The flexible body of the present invention preferably has a flexibleanterior wall connecting the upper and lower portions of the U-shapedbody, thereby providing a spring quality to the U-shaped body forflexibly resisting extreme FSU extension. This flexible anterior wall ispreferably shaped to conform with the opposed surfaces of the opposingspinous processes (as shown in FIG. 1 c). This quality also insures thegrip of the implant and reduces unwanted stresses upon the flexiblebody. In some embodiments, the thickness of the distal wall is greaterthan the thickness of the posterior portions.

Now referring to FIGS. 2 a and 2 b, there is provided an interspinousimplant 51 for insertion between adjacent spinous processes, the implantcomprising:

-   a) a flexible U-shaped body 61:    -   i) an upper portion 71 having an upper surface 73 adapted to        bear upon an upper spinous process and a lower surface 75,    -   ii) a lower portion 81 having a lower surface 83 adapted to bear        upon a lower spinous process and an upper surface 85,    -   iii) a flexible distal wall 91 connecting the upper and lower        portions, and-   b) a cushion element 95 having an upper surface 97 and a lower    surface 99,    wherein the lower surface of the upper portion of the flexible body    comprises a porous coating 98 thereon, and wherein the upper surface    of the cushion element is interdigitated with the porous coating.

In use, the cushion element provides a dampening effect upon naturalextension. The interdigitated nature of the cushion bond reducesmigration concerns.

In some embodiments, the bonding covers substantially the entire extentof the inner surface of the U-shaped body (i.e., the upper surface ofthe cushion is bonded to the lower surface of the upper posteriorportion, the anterior surface of the cushion is bonded to the posteriorsurface of the flexible anterior wall, and the lower surface of thecushion is bonded to the upper surface of the lower posterior portion).

Now referring to FIG. 2 c, in some embodiments, the bonding covers onlythe posterior portions of the inner surface of the U-shaped body (i.e.,the lower surface of the upper posterior portion, and the upper surfaceof the lower posterior portion, but not the posterior surface of theflexible anterior wall). The partial coverage of this embodimentprovides an amount of stress relief to the cushion-U-shaped bodyinterface.

The cushion element of FIGS. 2 a-2 b is preferably made of anelastomeric material, more preferably a polyolefin rubber or carbonblack reinforced polyolefin rubber. The hardness of the elastomericcushion element is preferably between 56 and 72 shore A durometer. Theultimate tensile strength of the cushion element is preferably greaterthan 1600 psi. The cushion element preferably has an ultimate elongationgreater than 300% using the ASTM D412-87 testing method, and a tearresistance greater than 100 psi using the ASTM D624-86 testing method.Although the cushion element is preferably a polyolefin rubber, it canbe made of any elastomeric material that simulates the response of thenatural ligaments.

Still referring to FIG. 2 a, a porous coating 98 is provided as theinner surface of the U-shaped body. The porous coating provides anopportunity for the cushion element to interdigitate with the porouscoating, and so obtain a greater amount of surface contact between theU-shaped body and the cushion, thereby achieving a lower maximum stress.In some embodiments, the coating covers the entire extent of the innersurface of the U-shaped body (i.e., the upper surface of the cushion isbonded to the lower surface of the upper posterior portion, the anteriorsurface of the cushion is bonded to the posterior surface of theflexible anterior wall, and the lower surface of the cushion is bondedto the upper surface of the lower posterior portion). Preferably, thecoating comprises a layer of small spherical particles or beads.

In some embodiments, the coating covers only the posterior portions ofthe inner surface of the U-shaped body (i.e., the lower surface of theupper posterior portion, and the upper surface of the lower posteriorportion, but not the posterior surface of the flexible anterior wall).

In some embodiments, a coating may also be applied to the superior sideof the upper portion and the inferior side of the lower portion topromote bony ingrowth and osteointegration. In some embodiments thereof,and the coating may include beads, and may have osteobiologic componentssuch as hydroxyapatite or tricalcium phosphate.

The present inventors have noted that there may be a need to correct therange of motion (ROM) provided by a motion disc after the motion dischas been implanted and there is need to change the load transferredthrough the facet joints to alleviate pain and facet joint degeneration.

For example, because implantation of spinal prostheses is an inexactprocedure, there may be times when implantation provides too much or toolittle motion. For example, in some implantation procedures, damage tothe anterior longitudinal ligament (ALL) is contemplated. Because theALL in its physiologic form restricts the flexion/extension range of thenatural disc, damage to it may provide the implanted disc with anunacceptably large range of motion (ROM) in flexion and extension. Thisoverly large ROM is problematic because it produces atypical loads uponthe facet joints as well as the adjacent intervertebral discs, therebyleading to premature degeneration of those facet joints andintervertebral discs. Accordingly, there may be a need topost-operatively correct the ROM of the implant in order to fine tunethe ROM.

In another example, an implanted disc has an acceptable ROM at the timeof implantation, but the patient undergoes typical aging so that thepatient's normal range of motion decreases over time. In this case, itmay be desirable to decrease the implant ROM so that it corresponds withthe patient's natural decreased ROM.

Accordingly, there may be a need to post-operatively correct the ROM ofthe implant in order to adjust the implant ROM to the new needs of thepatient.

The implant of the present invention is advantageous because it can beinserted into the spine at a first stiffness, and then adjusted to asecond stiffness to meet the needs of the particular patient.

In a first preferred embodiment, the stiffness of the implant isadjusted post-operatively in order to fine tune the implant to thesurgical needs of the patient.

In a second preferred embodiment, the stiffness of the implant isadjusted in order to fine tune the implant to the changing post-surgicalneeds of the patient.

In many embodiments, the stiffness of the implant is increased in orderto reduce the ROM of a functional spinal unit (FSU).

In some embodiments, the implant further comprises a compression spring,and the overall stiffness of the implant is changed by adjusting thelength of the compression spring. Now referring to FIGS. 3 a-3 b, insome embodiments, there is provided an interspinous implant 301 forinsertion between adjacent spinous processes, the implant comprising:

-   -   a) a flexible outer shell 311 comprising:        -   i) an upper posterior portion 315 adapted to bear upon an            upper spinous process,        -   ii) a lower posterior portion 321 adapted to bear upon a            lower spinous process,        -   iii) a flexible anterior wall 325 connecting the upper and            lower posterior portions,    -   b) a compression spring 341 having an upper portion 343 and a        lower portion 345, the upper portion of the compression screw        being attached to the upper posterior portion of the flexible        outer shell, and    -   c) a worm screw 351 having a lower portion 353 connected to the        lower posterior portion and an upper portion 355 contacting the        lower portion of the compression spring.        In this particular embodiment, the upper portion of the worm        screw comprises a cup 357 having an annular sidewall 359        extending upward. The lower end portion of the compression        spring is not rigidly attached to the cup, but rather sits        freely in the annulus and bears against the cup. Containment by        the cup allows the upper end of the worm screw to simply bear        against the lower end of the spring without requiring rigid        connection thereto.

In use, actuation of the worm screw causes inner thread 363 of the wormscrew to turn relative to the outer cylinder 361 of the worm screw. Theouter cylinder 361 responds by moving axially upward, thereby forcingcompression of the compression spring, and increasing the effectiveresistance of the device to axial compression.

Now referring to FIGS. 4 a-4 c, in some embodiments, there is providedan interspinous implant 401 for insertion between adjacent spinousprocesses, the implant having an implant stiffness and comprising:

-   -   a) a flexible outer shell 411 having a shell stiffness and        comprising:        -   i) an upper surface 415 adapted to bear upon an upper            spinous process,        -   ii) a lower surface 421 adapted to bear upon a lower spinous            process,        -   iii) an arcuate anterior wall 425 connecting the upper and            lower surfaces, and        -   iv) an arcuate posterior wall 431 extending between the            upper and lower surfaces,    -   b) compliant side walls 451,453, extending between the upper and        lower surfaces, and    -   c) an inner core 441 (such as a hydrogel) contained within the        shell, wherein the inner core has an adjustable stiffness.

When it is desired to decrease the range of motion (“ROM”) of thefunctional spinal unit (“FSU”), the stiffness of the core material maybe increased, thereby increasing the stiffness of the implant and itsresistance to an axial load. The resulting increase in the stiffness ofthe interspinous implant provides a more substantial resistance toextension, thereby desirably decreasing the ROM of the FSU to correspondwith the needs of the patient.

Similarly, when it is desired to increase the range of motion (“ROM”) ofthe functional spinal unit (“FSU”), the stiffness of the core materialis decreased, thereby decreasing the stiffness of the implant and itsresistance to an axial load. The resulting decrease in the stiffness ofthe interspinous implant reduces resistance to extension, therebydesirably increasing the ROM of the FSU to correspond with the needs ofthe patient.

The implant of this embodiment of the present invention also has aflexible posterior wall extending between the upper and lower portionsof the U-shaped body. This posterior wall is preferably arcuate andpreferably connects the upper surface of the lower portion and the lowersurface of the upper portion of the U-shaped body to form asubstantially oval body (as shown). In this condition, the posteriorwall provides substantial closure to the U-shaped body. Accordingly,adjustment of the stiffness of the core material residing within theouter shell increases or decreases the stiffness of the implant.

The compliance of the sidewalls is selected to correspond with the levelof resistance desired by the implant. For example, in some embodiments(as in FIGS. 4 a-4 c,) the sidewalls are very thin and may be made of avery flexible material, such as a plastic weave. In these embodiments,the high compliance of the sidewalls will allow the core material tobulge laterally in response to an axial load, thereby tempering theresistance provided by the core material to the axial load.

In other embodiments, however, the sidewalls can be made of metal, andeven be integral with the outer shell. In these embodiments, thesidewalls will be flexible but more rigid than a plastic membrane. Inthese embodiments, the relative rigidity of the sidewalls will not allowthe core material to bulge significantly laterally, thereby augmentingthe resistance provided by the core material to the axial load.

Preferably, the core is a fluid material contained within the cavity ofthe shell and is made of a material having a quality whose adjustmentwill produce a change in the stiffness of the implant. When thestiffness of the core is adjusted, the overall stiffness of the implantcorrespondingly changes. In some embodiments, the core has a firststiffness and contributes between 10 and 20% of the overall initialstiffness of the implant. In such embodiments, the stiffness of the coreis increased to a second stiffness that increases the overall initialstiffness of the implant up to at least 40% to provide an adjustedimplant stiffness of at least 300 N/mm, and more preferably at least 500N/mm. When the implant stiffness is in this range, the implant can byitself provide sufficient stiffness to reduce the extension of a normallumbar FSU to less than 7 degrees, preferably less than 5 degrees.

Preferably, the core material is selected to be sensitive to an externalstimulus, which, when applied, stimulates the core material to adjustits stiffness from a first stiffness to a second stiffness. In someembodiments, the stimulus stimulates the core to increase its stiffness.In some embodiments, the stimulus stimulates the core to lower itsstiffness.

Preferably, the core material is sensitive to a stimulus selected fromthe group consisting pH, light, and electric current.

In preferred embodiments, the core material comprises a hydrogel. Inpreferred embodiments, the hydrogel undergoes expansion when stimulatedby a decreased pH. The resulting expansion of the core materialincreases the stiffness of the core, thereby increasing the stiffness ofthe implant and providing increased resistance to extension by the FSU.In some embodiments, the hydrogel is selected from ionic polymersdisclosed in US Published Patent Application No. 2002/0039620, thespecification of which is incorporated by reference in its entirety. Insome embodiments, the hydrogel is selected from ionic polymers disclosedin U.S. Pat. No. 6,475,639, the specification of which is incorporatedby reference in its entirety.

When pH is selected as the stimuli, in some embodiments, an acid or abase is introduced into the core material from an ex vivo source. Forexample, the acid or base can be administered subcutaneously via ahypodermic needle and introduced into the core material through a fluidport 455. The provision of a fluid port provides the surgeon with theflexibility to selected the amount of acid or base needed to suit theneeds of the patient.

In other embodiments in which pH is selected as the stimuli, the implantfurther comprises a container that individually houses and sequestersthe acid or base from the core material. For example, the acid or basecan be sequestered in a valved, separate compartment within the shellthat is in fluid connection with the cavity housing the core material.The valve is opened (for example, by telemetry), the acid or base entersthe cavity housing the core material and mixes with the core material.The resulting pH change causes a change in the specific volume of thecore material, thereby increasing or decreasing the stiffness of thecore material and the overall implant. The advantage of this embodimentis that the stiffness of the implant is changed through a completelynon-invasive technique.

In some embodiments (not shown), the device could be made of a shapememory metal having a relatively flexible property during themartensitic phase and a relatively stiff property in the austeniticphase. In one embodiment, this memory metal device could be implanted inits flexible martensitic phase. If the clinician desires to increase thestiffness of the implant, the clinician could raise the temperature ofthe device (by heating) to a temperature above its austenitic phase,thereby increasing the stiffness of the device and increasing itsresistance to an axial compressive load.

In some embodiments of the present invention, the implant furthercomprises smart features for helping the surgeon monitor and react tothe changing conditions of the implanted device.

In some embodiments, a sensing means is also used with the implant ofthe present invention. This sensing means analyzes physicalsurroundings. Its purpose is to identify when a significant change hasoccurred which could warrant adjusting the stiffness of the implant. Thesensor can be contained within the implant, or provided as a stand aloneentity.

In some embodiments, a reporting means for reporting the findings of thesensors to an ex vivo source is also used with the implant of thepresent invention. The reporter can be contained within the implant, orprovided as a stand alone entity.

In some embodiments, a receiver for receiving ex vivo-generatedinformation is also used with the implant of the present invention. Thereceiver can be contained within the implant, or provided as a standalone entity.

In some embodiments, the implant comprises two shells having flexibleanterior walls extending in the same direction, wherein the stiffness isadjusted by adjusting the distance between the respective flexibleanterior walls. Now referring to FIG. 5 a, there is provided aninterspinous implant 501 for insertion between adjacent spinousprocesses, the implant comprising:

-   -   a) a flexible outer shell 511 comprising:        -   i) an upper posterior portion 515 adapted to bear upon an            upper spinous process and having a lower end 517 having a            first set of teeth 519,        -   ii) a lower posterior portion 521 adapted to bear upon a            lower spinous process and having a upper end 522 having a            second set of teeth 523,        -   iii) a flexible anterior wall 525 connecting the upper and            lower posterior portions of the flexible outer shell,    -   b) a flexible inner shell 551 comprising:        -   i) an upper posterior portion 555 having an upper end 556            having a third set of teeth 557 engaged in the first set of            teeth,        -   ii) a lower posterior portion 571 having a lower end 573            having fourth set of teeth 577 engaged in the second set of            teeth,        -   iii) a flexible anterior wall 575 connecting the upper and            lower posterior portions.

In use, the implant of FIG. 5 a is implanted into the interspinous voidso that the opposing sets of teeth of the inner and outer shells areengaged to the opposed spinous processes, thereby providing a secureimplant and defining a distance between the anterior walls D₁ of theinner and outer shells. If the clinician desires to change the stiffnessof the implant, then the clinician may alter the distance D between theanterior walls of the inner and outer shells. Reducing the distance Dbetween the anterior walls will cause a decrease in the stiffness of theimplant, while increasing the distance D between the anterior walls willcause an increase in the stiffness of the implant.

Now referring to FIG. 5 b, when the clinician desires to decrease thestiffness of the implant of FIG. 5 a, the clinician can use a pair offoreceps (not shown) to engage the slots 581 provided on the upper andlower posterior portions of the inner shell. Providing a clamping forcethrough the foreceps squeezes together the posterior portions of theinner shell, thereby disengaging the respective pairs of teeth. Theclinician can then move the disengaged inner shell anteriorly by apredetermined distance to a second position (shown in shadow), therebydecreasing the distance between the anterior walls to a smaller distanceD₂ and lowering the stiffness of the implant.

In other embodiments, the slots of the implant of FIGS. 5 a and 5 b arereplaced within other means for adjusting the distance D between theflexible anterior walls of the inner and outer shells. For example, insome embodiments, a set screw or a worm gear may be provided on theimplant to alter the distance D, thereby adjusting the stiffness of theimplant.

Therefore, in accordance with the present invention, there is providedan interspinous implant for insertion between adjacent spinousprocesses, the implant comprising:

-   -   a) a flexible outer shell comprising:        -   i) an upper posterior portion adapted to bear upon an upper            spinous process and having a lower end having a first set of            teeth,        -   ii) a lower posterior portion adapted to bear upon a lower            spinous process and having a upper end having a second set            of teeth,        -   iii) a flexible anterior wall connecting the upper and lower            posterior portions of the flexible outer shell,    -   b) a flexible inner shell comprising:        -   i) an upper posterior portion having an upper end having a            third set of teeth engaged in the first set of teeth,        -   ii) a lower posterior portion having a lower end having            fourth set of teeth engaged in the second set of teeth,        -   iii) a flexible anterior wall connecting the upper and lower            posterior portions.

In preferred embodiments, the implant of the present invention is usedposteriorly in conjunction with a motion disc inserted within the discspace of the anterior portion of the spinal column. For example, in someembodiments, the implant of the present invention is used in conjunctionwith a motion disc having a large range of motion (“ROM”). Variousmotion discs are described by Stefee et al. in U.S. Pat. No. 5,071,437;Gill et al. in U.S. Pat. No. 6,113,637; Bryan et al. in U.S. Pat. No.6,001,130; Hedman et al. in U.S. Pat. No. 4,759,769; Ray in U.S. Pat.No. 5,527,312; Ray et al. in U.S. Pat. No. 5,824,093; Buttner-Janz inU.S. Pat. No. 5,401,269; and Serhan et al. in U.S. Pat. No. 5,824,094;all which documents are hereby incorporated herein by reference in theirentireties. The flexibility of the flexible body provides resistance toextreme extension, thereby restricting the motion disc to a more narrowand more physiologically desirable range of motion.

Therefore, in accordance with the present invention, there is provided akit for providing therapy to a functional spinal unit comprising anupper vertebrae having an upper spinous process, a lower vertebraehaving a lower spinous process, and a disc space therebetween, the kitcomprising:

-   a) an interspinous implant for insertion between adjacent spinous    processes, the implant comprising a flexible (preferably, U-shaped)    body comprising:    -   i) an upper posterior portion having an upper surface adapted to        bear upon an upper spinous process,    -   ii) a lower posterior portion having a lower surface adapted to        bear upon a lower spinous process, and    -   iii) a flexible (preferably arcuate) anterior wall connecting        the upper and lower portions, and-   b) an artificial disc adapted for insertion into the disc space.

We claim:
 1. An interspinous implant for insertion between adjacentspinous processes, the implant comprising: a) a flexible bodycomprising: i) an upper posterior portion having an upper surfaceadapted to bear upon an upper spinous process, ii) a lower posteriorportion having a lower surface adapted to bear upon a lower spinousprocess, iii) an arcuate, flexible anterior wall connecting the upperand lower portions, wherein the flexible anterior wall has a singlenarrowed width portion that allows the device to twist in response tospinal rotation, thereby more closely mimicking natural physiologicmovement, and b) a cushion element having an upper surface and a lowersurface, wherein the lower surface of the upper portion of the flexiblebody comprises a porous coating thereon, and wherein the upper surfaceof the cushion element is interdigitated with the porous coating, andwherein the upper and lower posterior portions each comprise a bearingsurface and opposed walls extending therefrom, the bearing surface andopposed walls of each posterior portion defining a bracket adapted toreceive a projecting portion of opposed spinous processes.
 2. Theimplant of claim 1 wherein the cushion element comprises an elastomer.3. The implant of claim 1 wherein the lower portion of the flexible bodycomprises an upper surface having the porous coating thereon, andwherein the lower surface of the cushion element is interdigitated withthe porous coating.
 4. The implant of claim 1 wherein the cushionelement has an anterior surface, the anterior surface being free of theflexible anterior wall.
 5. The implant of claim 1 wherein the cushionelement has an anterior surface, the anterior surface being bonded tothe flexible anterior wall.
 6. The implant of claim 1 wherein theflexible body has a height and a width, and the height of the flexiblebody is as least as great as its width.
 7. The implant of claim 6wherein the flexible body has a height of between 10 mm and 20 mm, and awidth of between 5 mm and 10 mm.